Re: [PATCH v3 2/3] mm/migrate_device.c: Copy pte dirty bit to page

[David Hildenbrand <david@redhat.com>]

On 26.08.22 17:55, Peter Xu wrote:
> On Fri, Aug 26, 2022 at 04:47:22PM +0200, David Hildenbrand wrote:
>>> To me anon exclusive only shows this mm exclusively owns this page. I
>>> didn't quickly figure out why that requires different handling on tlb
>>> flushs.  Did I perhaps miss something?
>>
>> GUP-fast is the magic bit, we have to make sure that we won't see new
>> GUP pins, thus the TLB flush.
>>
>> include/linux/mm.h:gup_must_unshare() contains documentation.
> 
> Hmm.. Shouldn't ptep_get_and_clear() (e.g., xchg() on x86_64) already
> guarantees that no other process/thread will see this pte anymore
> afterwards?

You could have a GUP-fast thread that just looked up the PTE and is
going to pin the page afterwards, after the ptep_get_and_clear()
returned. You'll have to wait until that thread finished.

Another user that relies on this interaction between GUP-fast and TLB
flushing is for example mm/ksm.c:write_protect_page()

There is a comment in there explaining the interaction a bit more detailed.

Maybe we'll be able to handle this differently in the future (maybe once
this turns out to be an actual performance problem). Unfortunately,
mm->write_protect_seq isn't easily usable because we'd need have to make
sure we're the exclusive writer.


For now, it's not too complicated. For PTEs:
* try_to_migrate_one() already uses ptep_clear_flush().
* try_to_unmap_one() already conditionally used ptep_clear_flush().
* migrate_vma_collect_pmd() was the one case that didn't use it already
 (and I wonder why it's different than try_to_migrate_one()).

-- 
Thanks,

David / dhildenb